As a web developer, you can call these native Windows APIs from JavaScript when your web app is running as an installed Windows 10 app (launched from the wwahost.exe process, rather than the browser). Additionally, your website running as Windows 10 app can also use the Microsoft Edge webview control to display remote and local web content and MSApp APIs for blob and stream handling, among other things.

Here are the top-level WinRT namespace areas available to all Windows 10 apps:

Provides an app with access to core system functionality and run-time information about its UI. Note: APIs in the Windows.UI.Xaml namespace are not available for JavaScript apps (which can use the equivalent web standards-based technologies).